got the D3100 and love it so far. Currently using the 18-55 and 55-300mm kit lenses.

These are the lenses I would like to purchase in the next 2 years:
35mm f1.8 DX Nikkor
8-16mm Sigma
30mm f/1.4 EX DC HSM Sigma (not sure as the Nikkor looks like a very sweet spot)
12-24mm AT-X 124 PRO DX II Tokina

Not sure about Tamron.

I'll be shooting landscape, cities and a bit of sport (rc-car racing). I believe that the 55-300mm will be sufficient for my needs. What do you think of those lenses?

Like you I bought the 55-300 first but then I moved on the the 50mm 1.8 and then a Tamron 10-25 3.5-4.5 and then the 35mm f1.8 DX Nikkor partly because I got fed up of manually focusing the 50mm while I was trying to shoot he kids and partly because I wanted more..

What I use the most out and about are the Tamron and the Nikkor 35mm and I generally take photo's of my kids, my dog and landscapes. I also use the Tamron for some fun wide angle portraits when I take shots around my work place for news and promos and stuff. I intend to do some mountain bike trails because that's why i bought my camera but I have no idea what I'm going to use, just going to take my bag and have fun.

Why are you not sure about the Tamron?

_________________"I'm playing all the right notes but not necessarily in the right order" - Eric Morecambe________________________________________D7000 and too many lenses.

I heard the same about Tamron but didn't have the cash for the Sigma, the only thing I'm not fussed about is a tiny bit of chromatic aberration at the larger aperture settings in highlights at the edges but this is fixed easily with the lens correction in lightroom.

I've looked through the Flickr groups and I couldn't see any difference for my needs, I'm no expert and only had a DSLR since December but the Tamron is a super fun lens and it was loads cheaper.

_________________"I'm playing all the right notes but not necessarily in the right order" - Eric Morecambe________________________________________D7000 and too many lenses.